Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper and lightly grease.
Spread macadamia nuts on a baking sheet and toast for 5–7 minutes until lightly golden and fragrant. Cool completely before chopping.
In a large bowl, whisk together melted butter, brown sugar, and granulated sugar for about 1 minute until glossy and smooth.
Whisk in eggs, egg yolk, and vanilla extract until smooth and slightly thickened.
Fold in flour, cornstarch, baking powder, and salt until just combined. Do not overmix.
Fold in chopped white chocolate and macadamia nuts. Reserve a small amount for topping if desired.
Spread batter evenly into prepared pan. Sprinkle reserved chocolate and nuts over the top. Add flaky sea salt if desired.
Bake for 28–32 minutes or until edges are golden brown and the center is slightly soft. A toothpick should come out with moist crumbs but no wet batter.
Cool completely before slicing. For cleaner cuts, chill for 15–20 minutes before slicing.